There are generally two forms of switchable magnetic devices. The first, and most popular, is the ferromagnetic-core electromagnet, which generates a magnetic field as current passes through a coil of wire surrounding a soft iron core. With proper driving electronics, electromagnets can efficiently transform electrical energy into magnetic energy. When electricity is no longer supplied to the magnet, the magnetic field dissipates (or appears in certain devices where an electromagnet is paired with a permanent magnet).
Some applications (e.g., retention and “pick-and-place” systems) are more suited to switchable magnets, but for various reasons (e.g., power, safety, etc.) electromagnets may not be preferred. For these situations, a second type of switchable magnetic device, which uses permanent magnets, may be employed. A typical device of this type has a primary permanent magnet with a flux path in direct contact with a ferrous target surface (usually steel). In the “engaged” state, the magnetic field from the primary magnet magnetizes the target surface and generates an attractive force. To disengage from the target surface, a secondary magnet may be positioned such that its magnetic field cancels that of the primary magnet at the surface, thereby eliminating or at least decreasing the attractive force between the magnet and the target material. There have been several variations to this approach (see, e.g., U.S. Pat. No. 6,707,360, the entire disclosure of which is incorporated by reference herein); however, they typically require a large amount of mechanical work to force the secondary magnet into the proper position to cancel the magnetic field of the primary magnet.
More generally, electromagnets and traditional switchable permanent magnets have several disadvantages. For example, electromagnets require electrical power, can be hazardous in lifting applications where power interruption is possible, and generally lose energy through joule heating in the coils. Traditional switchable permanent magnets are generally heavy and large, as they tend to require movement of multiple magnets relative to each other. Further, when a magnet is brought into contact with a target surface, the potential magnetic energy between them is lost to the surroundings as heat and noise during impact. This energy loss must be re-applied to the system to disengage the magnet, typically through mechanical work. This mechanical work may be substantial when the magnets are large.
Accordingly, a need exists for a system that maintains a strong attractive force to hold a target surface while minimizing the force required for disengaging the system from the target surface.